Switchgear and similar electrical isolation equipment are highly regulated by industry standards (e.g., IEEE, ANSI, etc.). Among other things, these standards require line-side (upstream) current-limiting fuses for voltage transformers (“VT”) used as sensors to monitor the condition and quality of power in medium voltage switchgear. Such fuses frequently resemble a tube having conductive terminals on each end and are typically mounted directly onto the voltage transformers in the switchgear cabinet. The industry standards also define the minimum clearance or spacing required in the absence of substantiating test documentation between exposed portions of adjacent conductors, such as adjacent electrical power buses, as well as from those conductors to ground for various voltage levels. The clearances are described in terms of direct or “strike” distances and linear surface or “tracking” distances.
Direct mounting of fuses onto the voltage transformers requires space in the switchgear. However, customer preferences for smaller and less expensive switchgear continue to push manufacturers toward ever smaller switchgear. As an example, for mature switchgear like the Masterclad™ series of medium voltage metal-clad switchgear from Schneider Electric USA, Inc., the voltage transformers and tubular fuses reside within a compartment that measures roughly 36 inches wide by 42 inches tall. On the other hand, smaller switchgear like the HVL/cb™ series of metal-enclosed switchgear from Schneider Electric USA require the voltage transformer and tubular fuse to fit within a compartment that is about half the size. This makes it difficult, if not impractical, to mount fuses directly onto voltage transformer in small footprint switchgears.
Similar challenges exist for other types of transformers in small footprint switchgears. For example, control power for breaker controls (e.g., relays, controllers, etc.) is often derived from the medium voltage switchgear primary circuit. The devices that convert power from the switchgear are commonly called control power transformers (“CPT”) and are generally larger than voltage transformers. As a result, it is especially difficult to mount fuses directly onto CPTs in small footprint switchgears. The above difficulty is compounded by the imperative also to comply with industry-standard clearance or performance requirements.
Thus, a need exists for a way to mount transformer fuses in small footprint switchgear and similar electrical isolation equipment where the space allocated for the fuses and transformers is limited while also complying with industry-standard performance requirements.